Extrusion molding of inorganic masses, such as ceramic-forming materials has been performed by passing a green body or composition, which is obtained by mixing and kneading molding aids such as organic binders, surfactants, lubricants, and plasticizers with inorganic materials, particularly ceramic-forming materials, through dies having a desired shape into a sheet, a bar, a hollow tube, a rectangular column, a hollow rectangular column, or a honeycomb structure. In particular, the extrusion-molded body in the form of ceramic honeycombs has been in use as a carrier for exhaust gas cleaning catalysts, filters, and heat exchangers in the fields of automobiles and various industries.
U.S. Pat. No. 4,551,295 relates to the extrusion of a plastic ceramic batch into articles of widely-differing profiles and shapes such as, for example, dinnerware and electrical insulators, and especially the extrusion of thin-walled honeycomb structures from ceramic batches capable of flowing or plastically deforming under pressure during extrusion. The U.S. patent discusses that a methyl cellulose, such as METHOCEL™ A4M cellulose ether having a viscosity of 4000 mPa·s, measured as a 2 wt. % aqueous solution at 20° C. according to Ubbelohde, has a low gelation temperature. Methocel A cellulose ethers have a methoxyl substitution of 27.5 to 31.5 weight percent or a D.S. of 1.64 to 1.92. According to FIG. 8 of the U.S. patent a sharp rise in extrusion pressure is observed when increasing the extrusion temperature in the range of 23-30° C. The patent discusses that the use of METHOCEL™ A4M when used with ceramic batches causes the plasticity thereof to be lost at about 30° C. Accordingly, an operating temperature above 30° C. would lead to a significant increase of power consumption as well as an increase of production defects like crack formation and other defects. Accordingly, this observed sharp rise in extrusion pressure requires energy-intensive cooling of the equipment to avoid an undue rise of the operating temperature. Moreover, the observed sharp rise in extrusion pressure typically also requires a low speed of the extruder screw, which results in a low production speed. A high speed of the extruder screw would lead to a higher friction and temperature evolution. The U.S. patent discusses that such rise in extrusion pressure is not observed when using as a binder/plasticizer METHOCEL™ F4M cellulose ether which is commercially available from The Dow Chemical Company and has a viscosity of 4000 mPa·s. METHOCEL™ F4M cellulose ether has a methoxyl substitution of 27.0-30.0 weight percent and a hydroxypropoxyl substitution of 4.0-7.5 weight percent. The U.S. patent suggests using a hydroxypropyl cellulose having a viscosity of 25,000-100,000 mPa·s, measured as a 2 wt. % aqueous solution at 20° C. according to Ubbelohde to permit the use of working temperatures greater than 35° C. in a twin screw extrusion apparatus. Unfortunately, for several applications the green modulus of extrusion-molded hollow bodies is not sufficient when METHOCEL™ F4M cellulose ether is used in their production, particularly when producing thin-walled honeycomb structures of large dimensions, such as cordierite honeycomb structural bodies which are used as a catalyst carrier for an exhaust gas purification catalyst in a combustion engine, such as an automobile engine. An insufficient green strength leads to an insufficient shape retention of the extruded structural body before baking or sintering, i.e., to easy deformation by its own weight or external forces.
U.S. Pat. No. 6,589,627 discusses the need for providing thin-walled honeycomb structures. Accompanying the increasing severity of the automobile controls in recent years, much research is spent on ways to reduce the thickness of cell walls of the cordierite honeycomb structural bodies. Reducing the thickness of cell walls of the cordierite honeycomb structural bodies reduces its heat capacity, which leads to a more rapid activation of the gas purification catalyst and to reduced hydrocarbon emissions immediately after the engine is started. However, when the cell walls of ceramic honeycomb structural bodies are to be reduced in the molding process, e.g., to less than 100 micrometers, the width of the extrusion channels becomes less and the resistance present when the molding material passes through the extrusion channels increases significantly. This increased frictional resistance requires an increased extrusion pressure, which leads to the above discussed disadvantages. It is possible to soften the molding material to reduce the frictional resistance, e.g., by adding additional water to the paste. Although fluidity is improved by softening the molding material, the shape retention of the extrusion molded honeycomb structural body decreases. To make it possible to reduce the thickness of the cell walls of cordierite honeycomb structural bodies by improving the fluidity of the molding material during passage through the extrusion mold while maintaining the shape retention of the molding material, U.S. Pat. No. 6,589,627 suggests cordierite honeycomb structural bodies that comprise as a binder a water-soluble cellulose ether that contains 27.5 to 31.5% methoxyl groups, does not contain hydroxypropoxyl groups and hydroxyethoxyl groups in an amount greater than 0.1%, and has a viscosity of a 2 wt.-% aqueous solution at 20° C. of less than 8000 centipoise, wherein the amount of the cellulose ether is 3 to 10 weight percent relative to the amount of the cordierite-converted starting material. Unfortunately, the extrusion pressure for cordierite compositions comprising such methyl cellulose is unduly high at temperatures of 50° C. or more.
U.S. Patent Application Publication 20100025897 discloses that cellulose ethers are used in compositions for ceramic extrusion-molded bodies as an organic binder because of their excellent plasticity, water retention and thermal gelation characteristics. U.S. '897 discusses that these cellulose ethers are disadvantageous in that they increase in frictional force with the die portion during extrusion molding and thus, the extrusion temperature rises owing to this frictional resistance. U.S. '897 suggests solving this problem by additionally incorporating a styrenesulfonate in ceramic compositions comprising a ceramic material and a water-soluble cellulose ether for extrusion molding to enable extrusion molding at high temperatures and thus increasing the extrusion molding speed.
The International patent application WO2007/047103 relates to the use of an organic binder system which comprises an organic lubricant, such as a monocarboxylic acid like stearic acid, grafted to a cellulose ether binder, such as methylcellulose or hydroxypropyl methylcellulose like Methocel™ A4M and 20-333 and Methocel™ F240 available from The Dow Chemical Company.
Unfortunately, the use of styrenesulfonate or of cellulose ether binders to which a monocarboxylic acid has been grafted significantly increases the costs of compositions for extrusion-molded bodies.
Accordingly, it would be desirable to provide new compositions for producing extrusion-molded hollow bodies, particularly for producing extrusion-molded hollow bodies having a honeycomb structure. It would be particularly desirable to provide new compositions for producing such extrusion-molded hollow bodies which can be extruded at a sufficiently low extrusion pressure to avoid the technical and economic disadvantages of high extrusion pressures that make operation of the extruders prematurely uneconomical due to high wear or high power costs. It would also be particularly desirable to provide new compositions for producing extrusion-molded hollow bodies, particularly extrusion-molded hollow bodies having a honeycomb structure, which have a sufficiently high wet green modulus to provide good shape retention even if the extrusion-molded hollow bodies have thin cell walls.